title: A single channel mutation alters agonist efficacy at 5-HT3A and 5-HT3AB receptors.
abstract: OBJECTIVE
  5-HT3 receptors are composed of 5-HT3A subunits (homomeric receptors), or combinations of 5-HT3A and other 5-HT3 receptor subunits (heteromeric receptors, the best studied of which are 5-HT3AB receptors). Here we explore the effects of partial agonists at 5-HT3A and 5-HT3AB receptors, and the importance of a channel-lining residue in determining the efficacy of activation.
  
  
  METHODS
  Wild type and mutant 5-HT3A and 5-HT3AB receptors were expressed in Xenopus oocytes and examined using two-electrode voltage-clamp, or expressed in HEK293 cells and examined using [(3)H]granisetron binding.
  
  
  RESULTS
  Dopamine, quipazine and VUF10166 were partial agonists at wild type 5-HT3A and 5-HT3AB receptors, with quipazine and VUF10166 causing a long-lived (>20 min) inhibition of subsequent agonist responses. At 5-HT3A receptors, mCPBG was a partial agonist, but was a superagonist at 5-HT3AB receptors, as it produced a response 2.6× greater than that of 5-HT. A T6'S substitution in the 5-HT3A subunit decreased EC50 and increased Rmax of dopamine and quipazine at both homomeric and heteromeric receptors. The greatest changes were seen with VUF10166 at 5-HT3AT6'SB receptors, where it became a full agonist (EC50 = 7 nM) with an EC50 58-fold less than 5-HT (EC50 = 0.4 μM) and no longer caused inhibition of subsequent agonist responses.
  
  
  CONCLUSIONS
  These results indicate that a mutation in the pore lining domain in both 5-HT3A and 5-HT3AB receptors alters the relative efficacy of a series of agonists, changing some (e.g. quipazine) from apparent antagonists to potent and efficacious agonists.
authors.affiliation: Department of Biochemistry, University of Cambridge, Cambridge, UK.
authors.forename: A J
authors.initials: AJ
authors.lastname: Thompson
authors.forename: S C R
authors.initials: SC
authors.lastname: Lummis
mesh_terms: D000818:Animals; D004298:Dopamine / Q000494:pharmacology; D054333:Drug Partial Agonism; D005260:Female; D017829:Granisetron / Q000378:metabolism; D057809:HEK293 Cells; D006801:Humans; D009154:Mutation; D009865:Oocytes; D018408:Patch-Clamp Techniques; D010880:Piperidines / Q000494:pharmacology; D011810:Quinoxalines / Q000494:pharmacology; D011814:Quipazine / Q000494:pharmacology; D044406:Receptors, Serotonin, 5-HT3 / Q000187:drug effects* / Q000235:genetics; D012701:Serotonin / Q000378:metabolism*; D058827:Serotonin 5-HT3 Receptor Agonists / Q000494:pharmacology*; D013997:Time Factors; D014982:Xenopus laevis
chemical_list: D004298:Dopamine; D017829:Granisetron; D010880:Piperidines; D011810:Quinoxalines; D011814:Quipazine; D044406:Receptors, Serotonin, 5-HT3; D012701:Serotonin; D058827:Serotonin 5-HT3 Receptor Agonists; C574388:2-chloro-(4-methylpiperazine-1-yl)quinoxaline
keywords: Cys-loop; binding; gating; heteromeric; ligand-gated ion channel; serotonin